Field of the Invention
This invention relates, in general, to an electrolytic process and, in particular, to a new and useful anodizing process which comprises an anodic oxidation of aluminum by means of an electrolyte of low dissolving power.
Anodizing is, as is known, an electrolytic process in use for some years, whereby a surface layer of oxide is formed, by the passage of an electric current in an acid electrolyte, using aluminum as the anode.
In the conventional anodizing processes, electrolytes of sulfuric acid have been used whose concentration varies between 180 and 240 g/ltr.
Likewise processes are known which use as electrolytes mixtures of sulfuric acid and oxalic acid, being commonly found in concentration of 150 g/ltr of sulfuric acid and 20-30 g/ltr of oxalic acid.
In the above mentioned processes, the rate of formation of the layer with the mentioned types of electrolyte ranges from 0.2 to 0.4 micron per minute, with a layer density of 2.4 to 3.0 g/cc.
Other types of electrolytes are the organic electrolytes used in integral anodizing which, while it is true, give higher growths, also have the disadvantages of greatly increasing the cost of the process due to the high cost of the electrolytes themselves and the high consumption of electric power due to the necessity of applying much higher current densities for the oxidation.
Later, when the electro-color process appeared, practically the same colors were obtained with that process but at a much lower cost.
Although its cost was low, the electrolyte used and with it the process were at a disadvantage because they could not compete with the previous ones with respect to layer thickness, rate of growth, hardness and resistance to abrasion, so that it became necessary to find an electrolyte of high yield or efficiency, good hardness and low dissolving power, and all this at the lowest possible cost.
Another disadvantage of the existing electrolytes is the necessity to use a cooling system, as the reactions involved in them are exothermic and their operating temperature range between 18.degree. to 20.degree. C.